Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions				3 Months Ended						9 Months Ended		201 Months Ended
	Nov. 04, 2015	Oct. 13, 2015	Oct. 12, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015
Subsequent Event [Line Items]
Average price per share of repurchased shares (in dollars per share)				$ 61.11			$ 77.67			$ 66.96	$ 78.43	$ 44.70
Dividends declared (in dollars per share)				$ 0.07			$ 0.06			$ 0.21	$ 0.18
Dividends payable date				Sep. 29, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 24, 2014	Mar. 25, 2014
Dividends record date				Sep. 15, 2015	Jun. 16, 2015	Mar. 17, 2015	Sep. 16, 2014	Jun. 10, 2014	Mar. 11, 2014
Quarterly Dividend [Member]
Subsequent Event [Line Items]
Dividends declared (in dollars per share)				$ 0.07	$ 0.07	$ 0.07	$ 0.06	$ 0.06	$ 0.06
Subsequent Event [Member]
Subsequent Event [Line Items]
Stock repurchased (in shares)	91,740
Average price per share of repurchased shares (in dollars per share)	$ 57.53
Subsequent Event [Member] | Restricted Stock Awards [Member] | November 2013 Grant [Member]
Subsequent Event [Line Items]
Number of restricted stock awards outstanding (in shares)			130,650
Non-cash charge expected in fourth quarter resulting from acceleration of lapse of restrictions			$ 5.7
Non-cash charge per diluted share expected in fourth quarter resulting from acceleration of lapse of restrictions (in dollars per share)			$ 0.13
Restricted stock awards still remaining outstanding after acceleration (in shares)			557,900
Subsequent Event [Member] | Shareholder Designated Charitable Contribution Program [Member]
Subsequent Event [Line Items]
Shareholder-designated contribution (in shares)		21,000,000
Expected expense related to Shareholder-designated charitable contribution program		$ 5.3
Expected expense related to Shareholder-designated charitable contribution program if all shares outstanding are registered in shareholders' name at record date		$ 6.4
Subsequent Event [Member] | Quarterly Dividend [Member]
Subsequent Event [Line Items]
Dividends declared (in dollars per share)	$ 0.07
Dividends declared date										Nov. 04, 2015
Dividends payable date										Dec. 29, 2015
Dividends record date										Dec. 15, 2015